This invention relates to an optical recording element which is capable of storing and retrieving information.
The modem information revolution has led to an ever increasing demand for data storage systems. As a case in point, CD and DVD disks represent successful high volume data storage technologies. One major advantage of these technologies is that reading or writing of data is accomplished by shining light on the disk so there is no physical contact between the media and the optical head. However, the total storage capacity of these disks is limited by the size of the smallest marks on the surface of the media that can be read by the wavelength of light employed. Many attempts have been made to develop data storage systems with progressively smaller marks. However, the required equipment is prohibitively expensive, and the data access rates tend to be unacceptably slow.
One way to increase the storage capacity of a medium is to record the information depthwise, rather than just on the surface. There could be used holography, two-photon optics, and similar methods for illuminating media in three dimensions, with the goal of producing marks in three dimensions, and thereby providing very high data capacity systems.
Bleaching and photoreactions (e.g., photochromicity) of organic dyes has also been used as a means to record optical data, both in a single layer in writeable CD-type media, and depthwise (dissolved in a bulk piece of polymer). However, a large amount of optical power is required in these systems to produce readable marks, therefore the rate of recording of such media is slow. Also, many photochromic systems also tend to fade over time.
Holographic recording has also been achieved by optically induced birefringence in suitable polymers, a process which relies on photo-alignment of the side chains within the polymers. Once again, a large amount of optical power is required, and this process is inefficient and slow. In addition, the fidelity of the recorded information may degrade with time since optically induced orientation tends to relax over time in polymers.
JP 2000-086588 discloses a recording medium using changes in circular dichroism based on the interconversion of chiral norbornadiene and quadricyclane derivatives. However, this technique requires enantiomerically enriched compounds that are difficult to synthesize. Furthermore, this application does not disclose the use of sensitizers for photoinduced electron transfer.
U.S. Pat. No. 5,759,721 discloses a holographic recording medium which uses a photopolymerization technique which can also be used for recording information optically in three dimensions.
There is a problem with this process, however, in that photopolymerization is usually accompanied by shrinkage of the material which is a consequence of the process of forming new chemical bonds among the constituents. Any dimensional changes that occur on writing limit the resolution that can be achieved, and reduce the data capacity of the medium. In addition, photopolymerization generally requires the use of low molecular weight reactants so that media made from these materials tend to be undesirably soft or sticky. Furthermore, the most common method of photopolymerization, free radical polymerization, is subject to interference by atmospheric oxygen which causes undesirable inconsistencies in the process.
It is an object of this invention to increase the storage capacity of a optical recording material. It is another object of this invention to provide an optical recording material which can record information depthwise, rather than just on the surface. It is still another object of this invention to provide an optical recording material which does not substantially change dimensions upon recording.
These and other objects are achieved in accordance with the invention which comprises an optical recording material which when exposed to actinic radiation produces a change in optical properties in the exposed regions, thereby providing a pattern of intelligence for storing and retrieving information, the recording material comprising:
a) a binder,
b) a reactant which is capable of undergoing a chemical transformation upon a one electron oxidation, thus causing the change in optical properties in the exposed regions, and
c) a sensitizer capable of absorbing actinic radiation to cause an initial one electron oxidation of the reactant.
In accordance with the invention, an optical recording material is obtained which possesses several advantages over the prior art.
1. The invention involves a photoinitiated chain reaction in a solid polymer that creates changes in the optical properties of the material. However, because our invention relies on photoisomerization rather than photopolymerization, the dimensional changes accompanying recording are negligible. (No new bonds are formed between molecules.)
2. The invention involves a recording process that is efficient in the use of light. Because the process involves a photoinitiated chain reaction, many new molecules are formed per photon absorbed (chemical amplification). A relatively large change in optical properties is obtained with only a small exposure to the recording beam.
3. The material of the invention is a simple, stable polymer, which can be conveniently fabricated into films and slabs.
4. The optical changes in the material of the invention are large, permanent, localized, and can easily be detected, forming the basis for an optical storage medium. The invention is especially suited to three dimensional optical data recording systems such as holography and two-photon optics.
5. Unlike free radical polymerization, cation radical rearrangements of the invention are not sensitive to molecular oxygen, and will not be subject to the inconsistent performance which is commonly observed for free radical photopolymerizations that are currently used in the art.
Any binder may be used in the invention provided it dissolves the reactant and sensitizer. Suitable binders include a monomeric glass as defined in U.S. Pat. Nos. 4,499,165 and 4,626,361, the disclosures of which are hereby incorporated by reference, such as sucrose octaacetate; or a polymeric material such as, for example, poly(alkyl methacrylate), poly(alkyl acrylate), polystyrene, polycarbonate, cellulose acetate or poly(vinyl butyral). In general, the binder should be optically transparent in the spectral region where the sensitizer absorbs, i.e., should not have significant absorption at the excitation wavelengths, and should not interfere with the chemical transformation of the reactant. The binder may also contain a plasticizer, a preservative, etc.
The optical recording element of the invention may be in the form of a self-supporting slab or disk. It may also be coated on a support such as poly(ethylene terephthalate), poly(ethylene naphthoate), polystyrene, cellulose acetate, inorganic supports such as glass, quartz, silicon, etc. In a preferred embodiment, the support is a polyester or glass.
The surface of the substrate may be treated in order to improve the adhesion of the recording layer to the support. For example, the surface may be corona discharge treated prior to applying the optical recording material. Alternatively, an under-coating or subbing layer, such as a layer formed from a halogenated phenol or a partially hydrolyzed vinyl chloride-vinyl acetate copolymer, can be applied to the surface of the support.
The recording layer thickness may range from about 1 xcexcm to about 1 cm, preferably from about 100 xcexcm to about 1000 xcexcm.
As noted above, the reactant used in the invention is capable of undergoing a chemical transformation upon a one electron oxidation, thus causing the change in optical properties in the exposed regions of the optical recording material. Such compounds undergo a photoinduced cation radical rearrangement to product species, a process which defines the recording event. With the product formation, there are accompanying changes in optical characteristics such as refractive index, fluorescence properties, or absorption spectrum. No new chemical bonds are formed between individual reactant molecules, therefore, there are negligible dimensional changes in the media during the recording event.
The reactant is usually present in a relatively high concentration. In a preferred embodiment, the reactant comprises from about 1 to about 50% by weight of said material, the sensitizer comprises from about 0.001 to about 10% by weight of the material, with the balance being binder.
The chemical transformation of the reactant is an isomerization including reactions such as cyclizations, cycloadditions and cycloreversions. General examples of such transformations are the interconversion between 1a and 1b or 2a and 2b. 
R in the formulas above and below can be H; a substituted or unsubstituted alkyl or alkoxy group having from about 1 to about 12 carbon atoms, preferably 1-3 carbon atoms, such as methyl, ethyl, isopropyl, butyl, etc; a cyano or a carboxylate group; a substituted or unsubstituted aryl group having from about 6 to about 18 carbon atoms, such as phenyl, naphthyl, phenanthryl, anthryl, etc.; a substituted or unsubstituted heteroaromatic group such as furyl, thienyl, pyridyl, benzofuryl, benzotbienyl, etc. Substituents on the aryl or heteroaryl groups include, for example, one or more alkyl, aryl, alkoxyl, aryloxyl, thioalkyl, thioaryl groups etc. In addition, some or all of the substituents R can be joined together to form additional ring systems.
Examples of 1a/1b are: 
Examples of 2a/2b are: 
Specific examples of reactants 1a and 2a are shown in Table 1.
In a preferred embodiment, the reactant is selected so that its oxidation potential is less than that of its product, and that a suitably rapid isomerization can occur upon electron transfer to the sensitizer (see below). The compounds listed above possess these characteristics, but there may exist other (as yet unidentified) molecules that share the same properties, and that would function equally well or better than those listed.
The sensitizer used in the invention initiates the chemical transformation of the reactant. The sensitizer must be capable of oxidizing the reactant to a radical cation after the sensitizer has absorbed light (i.e., photoinduced electron transfer). There are two distinct classes of sensitizers which may be used in the invention.
In one embodiment, the sensitizer upon absorption of the actinic radiation is capable of accepting an electron from the reactant. Examples of such sensitizers include those shown in Tables 2 and 3.
In another embodiment of the invention, the sensitizer upon absorption of said actinic radiation fragments gives an oxidant capable of accepting an electron from the reactant. Examples of such sensitizers include those shown in Table 4.
To determine whether a sensitizer is capable of oxidizing the reactant to a radical cation after the sensitizer has absorbed light, reaction energetics may be used. There are three controlling parameters in reaction energetics: the excitation energy (ES*) and the reduction potential (ESred) of the sensitizing electron acceptor (S) and the oxidation potential (ERox) of the reactant (R), an electron donor. For these reactions to be energetically feasible, the energy of the excited state should be higher or only slightly lower than the energy stored in the primary product, the radical ion pair, Sxe2x88x92*R+*. 
The excitation energy of the sensitizer (electron acceptor) is conveniently determined from the midpoint of the normalized absorption and emission spectrum of S, if the reaction proceeds from the singlet excited state. However, if the reaction proceeds via the triplet state, then the triplet energy of S should be used as the excitation energy.
The energy of the radical ion pair, EIP, is given by Eq. 1, where xcex94 is an energy increment that depends on the medium polarity and ranges from nearly zero in highly polar media to ca. 0.3 eV in the least polar media. The electrochemical measurements in polar solvents such as acetonitrile or methylene chloride
EIP=ERoxxe2x88x92ESred+xcex94xe2x80x83xe2x80x83Eq. 1
Polymeric media tend to be low in dielectric constant, and as a result would not strongly solvate the radical ion pair. Thus, the energy increment xcex94 in Eq. 1 is expected to be near the maximum value, i.e., in the range of 0.2 to 0.3 eV.
Thus, sensitizing electron acceptors with excitation energy equal to or larger than the difference between the oxidation potential of the reactant and the reduction potential of the acceptor, (ERoxxe2x88x92ESred), will satisfy the energetic requirements of photoinitiating the reaction, Eq. 2.
ES*xe2x89xa7ERoxxe2x88x92ESredxe2x80x83xe2x80x83Eq. 2
It is more convenient to express the energetic requirements of the sensitizing acceptor relative to the donor in terms of a rearranged form of Eq. 2.
ES*+ESredxe2x89xa7ESoxxe2x80x83xe2x80x83Eq. 3
According to Eq. 3, for the reaction to be energetically feasible, the algebraic sum of the excitation energy of the sensitizer and its reduction potential should be approximately equal to or larger than the oxidation potential of the reactant.
For the specific example of the reactant hexamethyldewarbenzene, which has an oxidation potential of 1.59 V vs. SCE, numerous sensitizing acceptors, which meet the requirement of Eq. 3, can be used. Listed in Table 2 are some of the compounds that meet the requirements, namely having the sum of excitation energy plus reduction potential that is equal to or exceeds 1.59 eV, and are therefore useful with hexamethyldewarbenzene reactant.
In general, derivatives from many different compounds can be used as electron accepting sensitizers for various reactants, provided that the energetic requirements discussed above are satisfied. These potential sensitizers include: cyanoaromatics such as 1-cyanonaphthalene, 1,4-dicyanonaphthalene, 9,10-dicyanoanthracene, 2,9,10-tricyanoanthracene, 2,6,9,10-tetracyanoanthracene, aromatic anhydrides and imides such as 1,8-naphthylene dicarboxylic, 1,4,6,8-naphthalene tetracarboxylic, 3,4-perylene dicarboxylic, and 3,4,9,10-perylene tetracarboxylic anhydride or imide; condensed pyridinium salts such as quinolinium, isoquinolinium, phenanthridinium, acridinium salts; and pyryllium salts. Among useful sensitizers that involve the triplet excited state are carbonyl compounds such as quinones such as benzo-, naphtho-, anthro-quinones with electron withdrawing substituents (e.g., chloro and cyano). Ketocoumarins especially those with strong electron withdrawing moieties such as pyridinium can also be used as sensitizers.
Examples of the above sensitizers are shown in Table 3. These sensitizers can optionally contain substituents such as methyl, ethyl, tertiary butyl, phenyl, methoxy, chloro, etc. that may be included to modify properties such as solubility, absorption spectrum, reduction potential, etc.
2) Sensitization Via Photochemical Generation of a Radical Cation
In this approach, excitation leads to fragmentation of the sensitizer and the formation of an oxidizing radical cation. An example of this class of sensitizers is N-methoxyphenanthridinium, Eq. 4. 
In the above illustration, the sensitizer upon absorption of actinic radiation reacts to produce a fragment radical cation, the fragment radical cation then accepts an electron from the reactant, whereby the oxidation potential of the neutral fragment is greater than that of the reactant.